Fix of Improve Support material adhesion by base type interface layers

under soluble supports. #5823
Implemented as a pull request #5903 by @spiky2021
and reworked.

commit c7993e6
Author: spiky2021 <77010315+spiky2021@users.noreply.github.com>
Date:   Thu Feb 11 12:39:25 2021 +0100

    Base type interfaces for soluble interface supports

    At the moment soluble support material adhesion is weak due to sparse support layers under soluble support layers. I reported as issue #5823 with pictures, as well.
    I modified two methods to the SupportMaterial Class including their headers.
    The new methods add two base type interface layers to the support structure, in case the extruders are different and soluble support is choosen.
    Since it is conditionally activated, it in general doesn't need a GUI input. But a GUI option number of base interface layers may enabled users to adapt this feature to their needs.
    This is my second try to provide a pull request on this topic. Reset my fromer repository, because first I merged this and all other changes to my master and couldn't provide separate pull request anymore.

src/libslic3r/Format/3mf.cpp

@@ -710,7 +710,7 @@ namespace Slic3r {
 #endif // ENABLE_RELOAD_FROM_DISK_FOR_3MF
 
         for (const IdToModelObjectMap::value_type& object : m_objects) {
-            if (object.second >= m_model->objects.size()) {
+            if (object.second >= int(m_model->objects.size())) {
                 add_error("Unable to find object");
                 return false;
             }
@@ -1395,7 +1395,7 @@ namespace Slic3r {
     {
         // deletes all non-built or non-instanced objects
         for (const IdToModelObjectMap::value_type& object : m_objects) {
-            if (object.second >= m_model->objects.size()) {
+            if (object.second >= int(m_model->objects.size())) {
                 add_error("Unable to find object");
                 return false;
             }